U.S. patent application number 12/754491 was filed with the patent office on 2011-10-06 for medical device with charge leakage detection.
This patent application is currently assigned to PACESETTER, INC.. Invention is credited to Steven W. Badelt, Neal Forss, George I. Isaac, Gabriel A. Mouchawar, Lyle Frank Weaver.
Application Number | 20110245888 12/754491 |
Document ID | / |
Family ID | 44710538 |
Filed Date | 2011-10-06 |
United States Patent
Application |
20110245888 |
Kind Code |
A1 |
Badelt; Steven W. ; et
al. |
October 6, 2011 |
MEDICAL DEVICE WITH CHARGE LEAKAGE DETECTION
Abstract
A medical device (implantable or external) is provided that
comprises a power source, a charge storage member, a terminal
connector, a switch network, a controller and a leak detection
module. The charge storage member is configured to receive and
store energy from the power source. The terminal connector is
configured to be coupled to a lead to be implanted in a patient
proximate to tissue of interest. The switch network is electrically
disposed between the charge storage member and the terminal
connector. The switch network changes between open and closed
states to disconnect and connect the charge storage member and the
terminal connector. The controller controls storage of energy in
the charge storage member and delivery of stimulating pulses from
the charge storage member to the lead coupled to the terminal
connector. The leak detection module obtains a leakage measurement
by sensing at least one of i) a voltage potential of the charge
storage member and ii) current flow from the charge storage member.
The leak detection module compares the leakage measurement to a
leakage threshold to determine when the leakage measurement
satisfies the leakage threshold.
Inventors: |
Badelt; Steven W.; (Granada
Hills, CA) ; Mouchawar; Gabriel A.; (Valencia,
CA) ; Isaac; George I.; (Port Hueneme, CA) ;
Forss; Neal; (East Palo Alto, CA) ; Weaver; Lyle
Frank; (Woodside, CA) |
Assignee: |
PACESETTER, INC.
Sylmar
CA
|
Family ID: |
44710538 |
Appl. No.: |
12/754491 |
Filed: |
April 5, 2010 |
Current U.S.
Class: |
607/6 |
Current CPC
Class: |
H01G 9/14 20130101; A61N
1/3981 20130101; H03K 17/082 20130101; G01R 19/0092 20130101; A61N
1/3956 20130101; A61N 1/3975 20130101; A61N 1/3931 20130101 |
Class at
Publication: |
607/6 |
International
Class: |
A61N 1/39 20060101
A61N001/39 |
Claims
1. A medical device, comprising: a power source; a charge storage
member configured to receive and store energy from the power
source; a terminal connector configured to be coupled to a lead to
be implanted in a patient proximate to tissue of interest, a switch
network electrically disposed between the charge storage member and
the terminal connector, the switch network changing between open
and closed states to disconnect and connect the charge storage
member and the terminal connector; a controller to control storage
of energy in the charge storage member and delivery of stimulating
pulses from the charge storage member to the lead coupled to the
terminal connector; and a leak detection module to obtain a leakage
measurement by sensing at least one of i) a voltage potential of
the charge storage member and ii) current flow from the charge
storage member, the leak detection module compares the leakage
measurement to a leakage threshold to determine when the leakage
measurement satisfies the leakage threshold.
2. The device of claim 1, wherein the leakage detection module
further comprises a current sensor disposed between the charge
storage member and the switch network.
3. The device of claim 1, wherein the leakage detection module
further comprises a current sensor disposed between the switch
network and the terminal connector.
4. The device of claim 1, wherein the leakage detection module
further comprises a voltage sensor disposed at the charge storage
member to detect the voltage potential across the charge storage
member.
5. The device of claim 1, wherein the leakage threshold constitutes
a preset current range, the leak detection module identifying a
current leak when the current flow is outside of the preset current
range.
6. The device of claim 1, wherein the leakage threshold constitutes
a preset voltage range, the leak detection module identifying a
current leak when the voltage potential of the charge storage
member is outside of the preset voltage range
7. The device of claim 1, wherein the charge storage member is
configured to receive the energy during a refractory period of at
least a portion of the tissues of a heart.
8. The device of claim 1, wherein the controller controls the
charge storage member to receive the energy during a charging
period which is synchronized with at least one of an atrial event
and a ventricular event of a heart.
7. The device of claim 1, wherein the controller is configured to
synchronize a charging period of the charge storage member with an
R wave of the heart.
8. The device of claim 1, wherein the controller is configured to
sense at least one of the voltage potential and current
continuously.
9. The device of claim 1, wherein the controller is configured to
sense at least one of the voltage potential and current
intermittently.
10. The device of claim 1, wherein the controller is configured to
sense at least one of the voltage potential and current for a
preset charging period.
11. The device of claim 1, further comprising a current sensing
member that includes a diode disposed in parallel with a resistor
to sense the current flow to or from the switch network.
12. The device of claim 1, further comprising a current sensing
member that includes at least two diodes disposed in parallel with
a resistor, the diodes being disposed to allow current flow in
opposite directions, the resistor to sense the current flow to or
from the switch network.
13. The device of claim 1, wherein the controller decouples the
charge storage member from the power source upon detecting that the
leakage measurement exceeds the leakage threshold.
14. The device of claim 1 further comprising an electrode
operationally coupled to the charge storage member and the tissue,
the electrode configured to deliver the energy from the charge
storage member to the tissues, wherein the controller is configured
to operationally decouple the electrode from the charge storage
member upon detecting that the leakage measurement exceeds the
leakage threshold.
15. The device of claim 1, wherein the controller is configured to
issue at least one of a vibratory warning signal and an audible
warning signal upon detecting the leakage measurement exceeds the
leakage threshold.
16. A method of detecting energy leakage from a medical device,
comprising: initiating charge of a charge storage member in the
medical device; sensing at least one of i) a voltage potential of
the charge storage member and ii) current flow from the charge
storage member, to obtain a leakage measurement; comparing the
leakage measurement to a leakage threshold to determine when the
leakage measurement exceeds the leakage threshold; and identifying
energy leakage when the leakage measurement satisfies the leakage
threshold.
17. The method of claim 16, further comprising terminating charging
of the charge storage member upon identifying energy leakage, and
operationally decoupling the charge storage member from the tissue
upon identifying energy leakage.
18. The method of claim 16, further comprising operationally
decoupling the electrode from the charge storage member upon
identifying energy leakage.
19. The method of claim 16, further comprising determining a
refractory period of at least a portion of the tissue of interest
and timing the initiating operation to begin charging the charge
storage member during the refractory period.
20. The method of claim 16, further comprising charging the charge
storage member for a charging period ranging from about 20 msec to
about 50 msec. before the sensing operation.
21. The method of claim 16, further comprising synchronizing the
initiating operation with at least one of an atrial event and a
ventricular event of a heart.
22. The method of claim 16, further comprising synchronizing the
initiating operation with an R wave of the heart.
23. The method of claim 16, further comprising measuring at least
one of the voltage potential and the current for a preset period
continuously.
24. The method of claim 16, further comprising measuring at least
one of the voltage potential and the current for a preset period
intermittently.
25. The method of claim 16, wherein the sensing operation comprises
monitoring the current flowing in both directions between the
tissue of interest and the charge storage member.
26. The method of claim 16, wherein the comparing operation
comprises: obtaining at least one of an average voltage potential
and average current flow over a preset period; and comparing the at
least one of the averaged voltage potential and average current
flow to a preset voltage range and preset current range,
respectively.
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present invention relate generally to
medical devices that utilized charge storage members for treating
various cardiac, physiologic and neurologic disorders. More
particularly, embodiments of the present invention relate to
implantable or external medical devices with leakage detection
circuitry to detect leakage of energy to tissue and with leakage
prevention circuitry to take corrective action based upon detection
of energy leakage.
BACKGROUND OF THE INVENTION
[0002] Numerous medical devices exist today, including but not
limited to electrocardiographs ("ECGs"), electroencephalographs
("EEGs"), squid magnetometers, implantable pacemakers, implantable
cardioverter-defibrillators ("ICDs"), neurostimulators,
electrophysiology ("EP") mapping and radio frequency ("RF")
ablation systems, and the like (hereafter generally "implantable
medical devices" or "IMDs". IMDs commonly employ one or more
conductive leads that either receive or deliver voltage, current or
other electromagnetic pulses (generally "energy") from or to an
organ or tissue (collectively hereafter "tissue") for diagnostic or
therapeutic purposes.
[0003] Certain types of IMDs include internal charge storage
members, such as one or more capacitors. The charge storage members
are connected to a switch network also referred to as a bridge. The
bridge includes a network of transistors that are controlled by a
processor to open and close in different combinations to deliver
stored energy from the charge storage members to the tissue through
the electrodes.
[0004] However, the potential exists that electrical components
along the conductive path between the charge storage members and
the electrodes may experience electrical failure. For example, the
bridge may become damaged when a high voltage shock is delivered
over a lead that has a faulting electrode or conductor(s) therein.
When a lead undergoes a fault, the conductive wires within the lead
may be directly shorted to one another or may become directly
shorted to the housing of the IMD 10 (which also may serve as a
shocking electrode). When one or more electrodes are in a short
circuit condition, the current of a high voltage shock from the
charge storage members does not discharge into the normally
expected resistive load of tissue, such as the heart. Without the
resistive load of the tissue to absorb the current, a large voltage
potential builds up across the bridge. In this instance, the high
voltage potential from the capacitors is applied directly across
the bridge. While the transistors in the bridge are well suited to
carry high current, these transistors are not designed to withstand
large voltage potentials at high current. When a large voltage
potential is created across one or more of these transistors, this
may damage one or more of the transistors in the bridge.
[0005] Alternatively, a lead may be operating normally, but receive
a large voltage from an external source such as from an external
defibrillator. External defibrillation may induce over 1000 V on a
lead. This high voltage potential may also damage the transistors
in the bridge. When the transistors in the bridge damage, the
potential exists that the bridge can no longer isolate the charge
storage members from the tissue. Without electrical isolation, as
soon as the charge storage members begin to charge, current leaks
from the charge storage members to the tissue of interest. Current
leakage from high voltage energy storage capacitors of an IMD 10
may occur due to the reasons noted above as well as due to various
other reasons such as, for example, defective components in the
IMDs, components damaged during the handling process, electrical
overstress by the erroneous implantation of the IMDs into the
patient's heart by the surgeons, semiconductor contamination of the
switching devices of the IMD.
[0006] When the bridge experiences a failure, one or more of the
switching transistors may permanently enter an open circuit state
or a closed circuit state. When certain combinations of the
switching transistors fail in a closed circuit state, the potential
exist that the charge storage members become directly and
permanently connected to the connector terminals that are joined to
one or more electrodes. Thus, as soon as the IMD 10 begins to
charge the charge storage members, spurious current may flow from
the charge storage members through the lead and be delivered to the
tissue surrounding the electrodes. It is mandated by International
Standards that medical devices do not inject spurious current into
the patient beyond certain limits. First, spurious current flow may
promote electrode corrosion or electroplating on the electrodes.
Second, spurious current may stimulate the surrounding tissue at a
time when stimulation is not needed or desired.
[0007] IMDs may charge the energy storage capacitors periodically
even when a patient does not need therapy. For example, various
capacitors, such as the commonly employed aluminum electrolytic
capacitors, are typically charged to full voltage every couple of
months to prevent performance degradation. Whether energy is
actually required to perform capacitor reformation depends upon
whether the patient receives relatively frequent defibrillation
shocks. IMDs that do not periodically receive at least one
defibrillation shock may receive a periodic cycle of capacitor
reformation. IMDs that receive at least a defibrillation shock
every month or two; however, do not typically require such periodic
capacitor reformation because such capacitor reformation is
achieved automatically during the generation of the defibrillation
shocks.
[0008] In general, the amount of allowable leakage currents depends
on various design configurations of the implanted electrodes of the
IMDs such as implant positions of such electrodes, surface areas
thereof, and so forth. A current density for cardiac stimulation by
the direct contact electrodes has been determined to be about 1.5
mA/cm.sup.2, below which it is very unlikely for excitable cardiac
tissues to be stimulated. IMDs generally include large-area
electrodes for high voltage shock therapies as well as small-area
electrodes for pacing and sensing, where each has its own limit for
the allowable current leakage.
[0009] In accordance with certain standards, the allowable current
leakage from the high-voltage (HV) cardiac electrodes under normal
operating conditions is limited to 1 uA when the HV capacitors are
discharged and 10 uA when such capacitors are charged. When the
output switches of the IMDs leak electric current (e.g., through
various electrodes thereof) in an amount less than the foregoing
standards, cardiac tissues around the leaking electrodes do not
generally respond to such direct current and, thus, do not exhibit
unwanted excitation. Even under this circumstance, however, various
implanted electrodes of the IMDs can corrode, electroplate, and/or
otherwise degrade. Gradual degradation of such electrodes may
eventually lead to total destruction thereof, formation of open
circuit there around, and the like.
[0010] For example, when the electrodes implanted into the
patient's right ventricle are shorted to the case, an output switch
bridge or switch bank of the IMD 10 will be shorted out (HV energy
switches generally fail in the on state) and deliver electric
current directly into the surrounding cardiac tissues. Therefore,
in the event of any high voltage application such as in
cardioversion or defibrillation therapy, destroyed high-voltage
switch bank and/or bridges thereof the high voltage charger may
apply power of 4 watts directly to the surrounding tissues, thereby
potentially placing the patient in a hazardous situation.
[0011] Accordingly, there is a need to provide IMDs with leakage
current detection circuitry and circuitry to perform appropriate
mitigating action when leakage current is detected.
SUMMARY
[0012] In accordance with an embodiment, a medical device (external
or implantable) is provided that comprises a power source, a charge
storage member, a connector, a switch network, a controller and a
leak detection module. The charge storage member is configured to
receive and store energy from the power source. The connector is
configured to be coupled to a lead to be implanted in a patient
proximate to tissue of interest. The switch network is electrically
disposed between the charge storage member and the connector. The
switch network changes between open and closed states to disconnect
and connect the charge storage member and the connector. The
controller controls storage of energy in the charge storage member
and delivery of stimulating pulses from the charge storage member
to the lead coupled to the connector. The leak detection module
obtains a leakage measurement by sensing at least one of i) a
voltage potential of the charge storage member and ii) current flow
from the charge storage member. The leak detection module compares
the leakage measurement to a leakage threshold to determine when
the leakage measurement satisfies the leakage threshold.
[0013] Optionally, the leakage detection module may further
comprise a current sensor disposed between the charge storage
member and the switch network. The current sensor may be disposed
between the switch network and the connector. Optionally, the
leakage detection module may comprise a voltage sensor disposed at
the charge storage member to detect the voltage across the charge
storage member. The current sensing member includes a resistor
disposed in parallel with a diode to sense the current flow to or
from the switch network. The current sensing circuitry may include
at least two diodes disposed in parallel with the resistor, where
the diodes are disposed to allow current flow in opposite
directions. The resistor senses the current flow to or from the
switch network. The diode or diodes bypass the resistor in order to
allow for the delivery of shock current which is many orders of
magnitude as leakage currents.
[0014] Optionally, the leakage threshold may constitute a preset
current range. The leak detection module may identify a current
leak when the current flow is outside of the preset current range.
Optionally, the leakage threshold may constitute a preset voltage
range and the leak detection module may identify a current leak
when the voltage potential of the charge storage member is outside
of the preset voltage range.
[0015] In accordance with an embodiment, the charge storage member
is configured to receive the energy during a period of time in
which at least a portion of the tissues of a heart are in a
refractory condition. The controller controls the charge storage
member to receive the energy during a charging period which is
synchronized with at least one of an atrial event and a ventricular
event of a heart. The controller may be configured to synchronize a
charging period of the charge storage member with an R wave of the
heart.
[0016] Optionally, the controller may be configured to sense at
least one of the voltage potential and current flow continuously.
Alternatively, the controller may be configured to sense at least
one of the voltage potential and current flow intermittently.
Alternatively, the controller may be configured to sense at least
one of the voltage potential and current flow for a preset charging
period.
[0017] Optionally, the controller may decouple the charge storage
member from the power source upon detecting that the leakage
measurement exceeds the leakage threshold. The controller may
operationally decouple the electrode from the charge storage member
upon detecting that the leakage measurement exceeds the leakage
threshold. The controller is configured to issue at least one of a
vibratory warning signal and an audible warning signal upon
detecting that the leakage measurement exceeds the leakage
threshold.
[0018] In accordance with an alternative embodiment, a method is
provided for detecting energy leakage from a medical device
(external or implantable). The method comprises initiating charge
of a charge storage member in the medical device, and sensing at
least one of i) a voltage potential of the charge storage member
and ii) current flow from the charge storage member, to obtain a
leakage measurement. The method further includes comparing the
leakage measurement to a leakage threshold to determine when the
leakage measurement exceeds the leakage threshold and identifying
energy leakage when the leakage measurement satisfies the leakage
threshold.
[0019] Optionally, the method may further comprise terminating
charging of the charge storage member upon identifying energy
leakage, and operationally decoupling the charge storage member
from the tissue upon identifying energy leakage. The method may
further comprise operationally decoupling the electrode from the
charge storage member upon identifying energy leakage. The method
may further comprise determining a refractory period of at least a
portion of the tissue of interest and timing the initiating
operation to begin charging the charge storage member during the
refractory period.
[0020] Optionally, the method may comprise charging the charge
storage member for a charging period ranging from about 20 msec to
about 50 msec. before the sensing operation. The method
synchronizes the initiating operation with at least one of an
atrial event and a ventricular event of a heart. The method may
monitor the current flowing in both directions between the tissue
of interest and the charge storage member. The method may include
obtaining at least one of an average voltage potential and average
current flow over a preset period; and comparing at least one of
the averaged voltage potential and averaged current flow to a
preset voltage range and preset current range, respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a simplified, partly cut-away view of an exemplary
implantable medical device in electrical communication with at
least three leads implanted into a patient's heart.
[0022] FIG. 2 is a functional block diagram of the IMD of FIG.
1.
[0023] FIG. 3 illustrates a schematic diagram of a switch network
that may be located between the capacitors of a shocking circuit
and the terminals of the connector in accordance with an
embodiment.
[0024] FIG. 4 illustrates a circuit diagram of an exemplary charge
storage member and leakage detection system in accordance with an
embodiment
[0025] FIG. 5 illustrates a circuit diagram of an exemplary leakage
detection system in accordance with an alternative embodiment.
[0026] FIG. 6 illustrates a current leakage detection process
implemented by an IMD in accordance with an embodiment.
[0027] FIG. 7 illustrates an exemplary atrial cardiac event.
[0028] FIG. 8 illustrates an exemplary ventricular cardiac
event.
[0029] FIG. 9 illustrates a charge timing process implemented in
accordance with an embodiment.
[0030] FIG. 10 illustrates a post-leak assessment process performed
after leakage confirmation in accordance with an embodiment.
DETAILED DESCRIPTION
[0031] The following description is of a best mode presently
contemplated for practicing the present invention. This description
is not to be taken in a limiting sense but is made merely for the
purpose of describing the general principles of the invention. In
the description of the invention that follows, like numerals or
reference designators will be used to refer to like parts or
elements throughout. Although the following embodiments are
described principally in the context of pacemaker/defibrillator
unit capable of sensing and/or pacing pulse delivery, the medical
system may be applied to other IMD structures and external medical
devices. For example, embodiments may be implemented in an external
defibrillator, external programmer and the like. For example,
embodiments may be implemented in external defibrillators such as
described in U.S. Pat. Nos. 7,272,441; 7,257,440 and 6,990,373. As
further examples, embodiments may be implemented in leads for
devices that suppress an individual's appetite, stimulate the
patients nervous or muscular systems, stimulate the patient's brain
functions, reduce or offset pain associated with chronic conditions
and control motor skills for handicap individuals, and the
like.
[0032] A cardiac stimulation device will thus be described in
conjunction with FIGS. 1 and 2, in which the features included in
this invention could be implemented. It is recognized, however,
that numerous variations of such a device exist in which various
methods included in the present invention can be implemented
without deviating from the scope of the present invention.
[0033] FIG. 1 illustrates a IMD 10 in electrical communication with
a patient's heart 12 by way of three leads 20, 24 and 30 suitable
for delivering multi-chamber stimulation and/or shock therapy. To
sense atrial cardiac signals and to provide right atrial chamber
stimulation therapy, the device 10 is coupled to an implantable
right atrial lead 20 including at least one atrial tip electrode 22
that typically is implanted in the patient's right atrial
appendage. The right atrial lead 20 may also include an atrial ring
electrode 23 to allow bipolar stimulation or sensing in combination
with the atrial tip electrode 22.
[0034] To sense the left atrial and left ventricular cardiac
signals and to provide left-chamber stimulation therapy, the IMD 10
is coupled to a "coronary sinus" lead 24 designed for placement in
the "coronary sinus region" via the coronary sinus ostium in order
to place a distal electrode adjacent to the left ventricle and
additional electrode(s) adjacent to the left atrium. As used
herein, the phrase "coronary sinus region" refers to the venous
vasculature of the left ventricle, including any portion of the
coronary sinus, great cardiac vein, left marginal vein, left
posterior ventricular vein, middle cardiac vein, and/or small
cardiac vein or any other cardiac vein accessible by the coronary
sinus.
[0035] Accordingly, the coronary sinus lead 24 is designed to:
receive atrial and/or ventricular cardiac signals; deliver left
ventricular pacing therapy using at least one left ventricular tip
electrode 26 for unipolar configurations or in combination with
left ventricular ring electrode 25 for bipolar configurations;
deliver left atrial pacing therapy using at least one left atrial
ring electrode 27 as well as shocking therapy using at least one
left atrial coil electrode 28.
[0036] The IMD 10 is also shown in electrical communication with
the patient's heart 12 by way of an implantable right ventricular
lead 30 including, in this embodiment, a right ventricular (RV) tip
electrode 32, a right ventricular ring electrode 34, a right
ventricular coil electrode 36, a superior vena cava (SVC) coil
electrode 38, and so on. Typically, the right ventricular lead 30
is inserted transvenously into the heart 12 so as to place the
right ventricular tip electrode 32 in the right ventricular apex
such that the RV coil electrode 36 is positioned in the right
ventricle and the SVC coil electrode 38 will be positioned in the
right atrium and/or superior vena cava. Accordingly, the right
ventricular lead 30 is capable of receiving cardiac signals, and
delivering stimulation in the form of pacing and shock therapy to
the right ventricle.
[0037] FIG. 2 illustrates a simplified block diagram of the
multi-chamber IMD 10, which is capable of treating both fast
arrhythmia and slow arrhythmia with stimulation therapy, including
cardioversion, defibrillation, and pacing stimulation. While a
particular multi-chamber device is shown, this is for illustration
purposes only, and one of ordinary skill in the pertinent art could
readily duplicate, eliminate or disable the appropriate circuitry
in any desired combination to provide a device capable of treating
the appropriate chamber(s) with cardioversion, defibrillation,
and/or pacing stimulation.
[0038] The IMD 10 includes a housing 40 which is often referred to
as "can," "case," or "case electrode," and which may be
programmably selected to act as the return electrode for all
"unipolar" modes. The housing 40 may further be used as a return
electrode alone or in combination with one or more of the coil
electrodes 28, 36, or 38, for defibrillation shocking purposes. The
housing 40 further includes a connector having a plurality of
terminals 42, 43, 44, 45, 46, 48, 52, 54, 56, and 58 (shown
schematically and, for convenience, the names of the electrodes to
which they are connected are shown next to corresponding
terminals). As such, in order to achieve right atrial sensing and
stimulation, the connector includes at least one right atrial tip
terminal (RA TIP) 42 adapted for connection to the atrial tip
electrode 22. The connector may also include a right atrial ring
terminal (RA RING) for connection to the right atrial ring
electrode 23.
[0039] To achieve left chamber sensing, pacing, and/or shocking,
such a connector includes a left ventricular tip terminal (LV TIP)
44, a left ventricular ring terminal (LV RING) 25, a left atrial
ring terminal (LA RING) 46, and a left atrial shocking coil
terminal (LA COIL) 48, that are adapted for connection to the left
ventricular tip electrode 26, the left ventricular ring electrode
25, the left atrial ring electrode 27, and the left atrial coil
electrode 28, respectively.
[0040] To support right ventricular sensing, pacing, and/or
shocking, the connector may further include a right ventricular tip
terminal (RV TIP) 52, a right ventricular ring terminal (RV RING)
54, a right ventricular shocking coil terminal (RV COIL) 56, and an
SVC shocking coil terminal (SVC COIL) 58, which are adapted for
connection to the right ventricular (RV) tip electrode 32, the RV
ring electrode 34, the RV coil electrode 36, and the SVC coil
electrode 38, respectively.
[0041] At the core of the IMD 10 is a programmable microcontroller
60 that controls the various modes of stimulation therapy. The
microcontroller 60 typically includes a microprocessor, or
equivalent control circuitry, designed specifically for controlling
the delivery of stimulation therapy, and may include RAM or ROM
memory, logic and timing circuitry, state machine circuitry, and/or
I/O circuitry. Typically, the microcontroller 60 may have the
ability to process or monitor various input signals (data) as
controlled by a program code stored in a designated block of
memory.
[0042] FIG. 2 illustrates an atrial pulse generator 70 and
ventricular pulse generator 72 which generate stimulation pulses
for delivery by the right atrial lead 20, the right ventricular
lead 30, and/or the coronary sinus lead 24 via a switch 74. It is
understood that, to provide the stimulation therapy in each of the
four chambers of the heart, the atrial pulse generator 70 and the
ventricular pulse generator 72 may include, e.g., dedicated pulse
generators, independent pulse generators, multiplexed pulse
generators, and/or shared pulse generators. The atrial pulse
generator 70 and the ventricular pulse generator 72 are generally
controlled by the microcontroller 60 via appropriate control
signals 76 and 78, respectively, to trigger or inhibit the
stimulation pulses.
[0043] The microcontroller 60 may further include timing control
circuitry 79 which may be used to control timing of the stimulation
pulses such as, e.g., pacing rate, atrio-ventricular (AV) delay,
atrial interchamber (A-A) delay, and/or ventricular interchamber
(V-V) delay. Such timing control circuitry 79 may also be used to
keep track of the timing of refractory periods, noise detection
windows, evoked response windows, alert intervals, marker channel
timing, and so on.
[0044] The switch 74 includes a plurality of switches for
connecting the desired electrodes to the appropriate I/O circuits,
thereby providing complete electrode programmability. Accordingly,
the switch 74, in response to a control signal 80 from the
microcontroller 60, determines the polarity of the stimulation
pulses (e.g., unipolar, bipolar, cross-chamber, and the like) by
selectively closing the appropriate combination of switches. Atrial
sensing circuits 82 and ventricular sensing circuits 84 may also be
selectively coupled to the right atrial lead 20, coronary sinus
lead 24, and the right ventricular lead 30 through the switch 74,
for detecting the presence of cardiac activity in each of the four
chambers of the heart.
[0045] Accordingly, the atrial sensing circuit 82 and the
ventricular sensing circuit 84 may include dedicated sense
amplifiers, multiplexed amplifiers or shared amplifiers. The switch
74 determines the "sensing polarity" of the cardiac signal by
selectively closing the appropriate switches. In this way, the
clinician may program the sensing polarity independent of the
stimulation polarity.
[0046] Each of the atrial and ventricular sensing circuits 82, 84
preferably employs one or more low power, precision amplifiers with
programmable gain, automatic gain or sensitivity control, band-pass
filtering, and threshold detection circuit, to selectively sense
the cardiac signal of interest. The automatic sensitivity control
enables the IMD 10 to deal effectively with the difficult problem
of sensing the low amplitude signal characteristics of atrial or
ventricular fibrillation.
[0047] The outputs of the atrial sensing circuit 82 and ventricular
sensing circuits 84 may be connected to the microcontroller 60 for
triggering or inhibiting the atrial and ventricular pulse
generators 70 and 72, respectively, in a demand fashion, in
response to the absence or presence of cardiac activity,
respectively, in the appropriate chambers of the heart. The atrial
and ventricular sensing circuits 82 and 84, in turn, may receive
control signals over signal lines 86 and 88 from the
microcontroller 60, for controlling the gain, threshold,
polarization charge removal circuitry, and the timing of any
blocking circuitry coupled to the inputs of the atrial and
ventricular sensing circuits 82 and 84.
[0048] For arrhythmia detection, the IMD 10 includes an arrhythmia
detector 77 that utilizes the atrial and ventricular sensing
circuits 82 and 84 to sense cardiac signals, for determining
whether a rhythm may be physiologic or pathologic. As used herein,
"sensing" generally refers to the process of noting an electrical
signal, while "detection" generally refers to the step of
confirming the sensed electrical signal as the signal being sought
by the detector. As an example, "detection" applies to the
detection of both proper rhythms (i.e., "P wave" or "R wave") as
well as improper disrhythmias including arrhythmia and bradycardia
(e.g., detection of the absence of a proper rhythm).
[0049] The timing intervals between sensed events (e.g., P-waves,
R-waves, and depolarization signals associated with fibrillation
which are sometimes referred to as "F-waves" or "Fib-waves") are
then classified by the arrhythmia detector 77 by comparing them to
a predefined rate zone limit (e.g., bradycardia, normal, low rate
ventricular tachycardia, high rate ventricular tachycardia,
fibrillation rate zones, and so on) and various other
characteristics (e.g., sudden onset, stability, physiologic
sensors, morphology, and so on), in order to determine the type of
remedial therapy required (e.g., bradycardia pacing,
anti-tachycardia stimulation, cardioversion shocks or
defibrillation shocks, collectively referred to as "tiered
therapy").
[0050] Cardiac signals are also applied to the inputs of a data
acquisition system 90 which is depicted as an analog-to-digital
(ND) converter for simplicity of illustration. The data acquisition
system 90 is configured to acquire intracardiac electrogram (e.g.
IEGM) signals, convert the raw analog data into digital signals,
and store the digital signals for later processing and/or
telemetric transmission to an external device 102. The data
acquisition system 90 may be coupled to the right atrial lead 20,
the coronary sinus lead 24, and the right ventricular lead 30
through the switch 74. The data acquisition system 90 may sample
the cardiac signals across any pair of desired electrodes. The data
acquisition system 90 may be coupled to the microcontroller 60
and/or another detection circuitry and controlled by signal 92, for
detecting an evoked response from the heart 12 in response to an
applied stimulus, thereby aiding in the detection of "capture."
Detecting the evoked response during the detection window may
indicate that capture has occurred.
[0051] The microcontroller 60 may further be coupled to a memory 94
by a suitable data/address bus 96, wherein the programmable
operating parameters used by the microcontroller 60 are stored and
modified, as required, so as to customize the operation of the IMD
10 to suit the needs of particular patients. Such operating
parameters may define, e.g., stimulation pulse amplitude, pulse
duration, polarity of electrodes, rate, sensitivity, automatic
features, arrhythmia detection criteria, and/or the amplitude,
shape of waves, and/or vector of each stimulation pulse to be
delivered to the patient's heart 12 within each respective tier of
therapy.
[0052] The IMD 10 may additionally include a power source that may
be illustrated as a battery 110 for providing operating power to
all the circuits of FIG. 2. For the IMD 10 employing shocking
therapy, the battery 110 must be capable of operating at low
current drains for long periods of time, preferably less than 10
uA, and also be capable of providing high-current pulses when the
patient requires a shock pulse, preferably in excess of 2 A, at
voltages above 2 V, for periods of 10 seconds or more. The battery
110 preferably has a predictable discharge characteristic such that
elective replacement time can be detected. A physiologic sensor 108
detects motion of the IMD and thus, patient to determine an amount
of activity.
[0053] A patient warning signal generator 64 may be included in the
microcontroller 60 so that a patient or operator may be alerted to
a condition requiring medical attention. A condition warranting a
patient alarm may be related to operation of the IMD 10 or may be
related to a detected patient condition. For example, patient
warning systems have been proposed for alerting a patient to a
detected tachycardia and impending stimulation therapy delivery. In
accordance with one exemplary embodiment, the patient warning
generator 64 may be used to alert the patient or operator to
current leakage detection as will be described later. Exemplary
patient warning signals include a twitch sensation caused by
delivery of a stimulation pulse or burst of pulses delivered to
excitable tissue, or an audible warning sound, or a vibratory
warning signal.
[0054] The IMD 10 includes an impedance measuring circuit 112 which
is enabled by the microcontroller 60 by control signal 114. The
known uses for an impedance measuring circuit 112 include, but are
not limited to, lead impedance surveillance during the acute and
chronic phases for proper lead positioning or dislodgement;
detecting operable electrodes and automatically switching to an
operable pair in case dislodgement should occur; measuring
respiration or minute ventilation; measuring thoracic impedance for
determining shock thresholds; detecting when the device has been
implanted; measuring stroke volume; detecting opening of heart
valves, and so on. The impedance measuring circuit 112 is
advantageously coupled to the switch 74 so that any desired
electrode may be used.
[0055] The IMD 10 may be used as an implantable cardioverter
defibrillator (ICD) device by detecting the occurrence of an
arrhythmia, and automatically applying an appropriate electrical
stimulation or shock therapy to the heart aimed at terminating the
detected arrhythmia. To this end, the microcontroller 60 further
controls a shocking circuit 116 by way of a control line 118. The
shocking circuit 116 includes charge storage members, such as one
or more capacitors. The charge storage members are charged by the
battery 110 before delivering stimulating energy such as high
voltage shocks. The charge storage members deliver the stimulating
energy over positive and negative lines 55 and 57. The switch 74
includes a switch network 61 that is electrically disposed between
the positive and negative lines 55 and 57, and the appropriate
terminals of the connector 43. The switch network 61 changes
between open and closed states to disconnect and connect the charge
storage members and the connector 43.
[0056] A leak detection module 63 is provided at the controller 60
to obtain leakage measurements. The leakage measurements are
obtained by a leak sensing circuit 53 located proximate the switch
network 61. The leak sensing circuit 53 may be located upstream or
down stream of the switch network 61 depending in part on the type
of leak detection to be performed. The leak sensing circuit 53 may
sense a voltage potential across the charge storage member in the
shocking circuit 116. Alternatively, or in addition, the leak
sensing circuitry 53 may sense current flow through the switch
network 61 and thus from the charge storage member. The leak
sensing circuitry 53 provides a leakage measurement (denoted at
line 59) to the leakage detection module 63. The leak sensing
circuitry 53 may represent a voltage sensor, a current sensor a
power sensor, a combination thereof and the like.
[0057] Optionally, to detect leakage, the controller 60 may
interrupt a charging operation and measure a voltage potential
across the capacitors in the shocking circuit 116 over control line
118. The controller 60 passes this voltage measurement to the
leakage detection module 63 which determines whether the voltage
potential across the capacitors of the shocking circuit 116
corresponds to an expected charge pattern. Optionally, the voltage
potential may be measured without interrupting a charging operation
of the capacitors.
[0058] The controller 60 manages operation of the leak sensing
circuitry 53 to obtain sensor reads (e.g. sense) of at least one of
the voltage potential and current flow continuously and at all
times throughout operation. Alternatively, the controller 60
manages operation of the leak sensing circuitry 53 to obtain sensor
reads (e.g. sense) of at least one of the voltage potential and
current flow intermittently. As a further option, the controller 60
may cause sensing to occur periodically after a preset charging
period.
[0059] The leak detection module 63 compares the leakage
measurement 59 to a leakage threshold 51 to determine when the
leakage measurement 59 exceeds the leakage threshold 51. The
leakage threshold may be pre-programmed and/or programmable by a
physician using a programmer and the like. The leakage threshold
may constitute a preset voltage range with upper and lower limits.
The leak detection module 63 may identify current leakage when the
voltage potential of the charge storage member is outside of the
preset voltage range.
[0060] Optionally, the controller 60 may control an initial charge
period for the shocking circuit 116 such that the charge storage
member initially only receives the energy from the battery 110
during a period of time in which a desired portion of the tissue of
the heart is in a refractory state. For example, the charging
period may be synchronized to occur only during the atrial
refractory period. Alternatively, the charging period may be
synchronized to occur only during the ventricular refractory
period. As one example of a manner to synchronize the charging
period to a desired refractory period, the start time at which a
charging period is initiated may be set a predetermined number of
milliseconds after the occurrence of an R-wave.
[0061] The controller 60 may attempt to mitigate or correct for
leakage. For example, the controller 60 may decouple the charge
storage member of the charging circuit 116 from the battery 110
when the leakage detection module 63 detects a leakage measurement
that satisfies the leakage threshold (e.g. the leakage measurement
represents an amount of current that exceeds a maximum acceptable
amount of leakage current, or the leakage measurement is a voltage
potential that is beyond a threshold voltage that is expected
across the HV capacitors). The decoupling may occur by
disconnecting charge storage capacitors entirely or by disabling a
portion of firmware/software that initiates a charging operation.
When an electrode is operationally coupled to the charge storage
member and the tissue, the controller 60 operationally decouples
the electrode from the charge storage member when the leakage
measurement satisfies the leakage threshold. The controller may
also issue at least one of a vibratory warning signal and an
audible warning signal, from the patient warning system 64, upon
detecting that the leakage measurement satisfied the leakage
threshold (e.g. exceeds a maximum or falls below a minimum).
[0062] FIG. 3 illustrates a schematic diagram of a switch network
300 (such as network 61) that may be located between the capacitors
of a shocking circuit 116 and the terminals of the connector 43.
The switch network 300 includes high voltage positive and negative
nodes 302 and 304 that are connected to the positive and negative
terminals of one or more energy storage capacitors in the shocking
circuit 116. A group of switching transistors 316-320 is joined in
the H-bridge architecture with the positive and negative nodes 302
and 304. The transistors 316-320 are controlled by the controller
60 (FIG. 2) to change between open circuit and closed circuit
states. The transistors 316-320 connect and disconnect the positive
and negative nodes 302 and 304 to desired combinations of
electrodes, such as an RV electrode 308, a CAN electrode 312 and a
SVC electrode 310, which are located proximate tissue of interest
306 in a patient. The transistors 316-320 in the example of FIG. 3
represent IGBT transistors.
[0063] When the transistors 316-320 fail, these IGBT transistors
usually fail to a short state in which the source and drain are
shorted together when too much power must be dissipated by the
switch network 300. As noted above, high power dissipation may be
introduced when the lead, or one or more of the electrodes (e.g.
308, 310, 312), fails in a shorted state. High power dissipation
may also be necessary when an external defibrillator is utilized on
the patient which then causes a dielectric breakdown. When high
power is introduced across the switch network, it may destroy two
or more of the transistors 316-320. When opposing transistors 317
and 319 fail, this may create a closed circuit between the positive
and negative nodes 302 and 304 through transistors 317 and 319, and
through electrodes 308 and 312 to the tissue 306 of the patient.
Alternatively, when opposing transistors 316 and 318 fail, this may
create a closed circuit between the positive and negative nodes 302
and 304 through transistors 316 and 318, and through electrodes 308
and 312 to the tissue of the patient. Thus, the patient may be
exposed to leakage current that may induce fibrillation or other
reactions when the IMD is attempting to charge the capacitors. The
failure of the switch network 300 may create a complete short or a
near short connection (e.g., a connection exhibiting low impedance
such as less than 20 ohms) between the high voltage charge circuit
and the terminals of the connector 43 (FIG. 2).
[0064] When same-side transistors 316 and 317 fail, this may create
a closed circuit between the positive and negative nodes 302 and
304 through transistors 316 and 317. While a same-side transistor
failure may not introduce spurious current into the patient, a
same-side transistor failure may prevent the capacitors from
charging and drain the battery unduly earlier.
[0065] In accordance with at least one embodiment, the IMD 10
measures the voltage across the positive and negative nodes 302 and
304 of the switch network 300. The IMD 10 may measure the voltage
across the positive and negative terminals of one or more
capacitors in the charge storage member (in shocking circuit. For
example, with reference to FIG. 2, the IMD 10 may measure the
voltage potential across the positive and negative lines 55 and
57.
[0066] Various components of IMD 10 may be degraded, shorted, and
leak electric energy to the surrounding cardiac tissues.
Embodiments of the present invention provide systems and associated
methods to detect the leaking current, to issue visual or audible
warning signals to the patient or medical staff, and to take
remedial actions in order to minimize or prevent further leakage
there from. As an example, a leakage detection system and method
may be provided for detecting current flow leaking from the IMD 10
by measuring the voltages across pre-selected components of the IMD
10, rates of charge buildup in a capacitor and/or the rate of
charge depletion of the IMD 10 capacitors after a predetermined
waiting period, current flows through various components of the IMD
10, and similar other indicators. Upon detecting the leakage, The
IMD 10 may be configured to issue various vibratory and/or audible
warning signals and to take remedial actions to minimize further
leakage of energy or current by, e.g., eliminating the leakage
components from the network, terminating the charging process of
the charge generators or capacitors, and terminating the operation
of at least a portion of the IMD 10 until proper remedial action is
taken.
[0067] Various embodiments of a leakage detection and warning
system will now be described. It is recognized, however, that
numerous variations of such systems and methods exist without
deviating from the scope of the present invention. As noted above
in connection with FIG. 3, the controller 60 may be configured to
monitor voltage or voltage change over time in at least one
location of the IMD 10 and to detect current leakage based
thereon.
[0068] While embodiments described herein utilize IGBT transistors
in an H-bridge configuration, optionally embodiments of the present
invention may be implemented utilizing other types of circuits and
other switch bridge configurations. For example, an H-bridge switch
could be implemented utilizing field effect transistors (FETs),
silicon controlled rectifiers (SCRs), unijunction transistors
(UJTs), bipolar junction transistors (BJTs), relays or any
combination thereof. Optionally, vacuum tube switches, such as
triodes, tetrodes, pentodes, etc., could be used to form the switch
network. Optionally, the switch network may be implemented
utilizing a network of circuits in a configuration that differs
from an H-bridge.
[0069] FIG. 6 illustrates a current leakage detection process
implemented by the IMD 10 in accordance with an embodiment of the
present invention. The leakage detection process 600 may be
implemented in connection with a voltage measurement circuit that
measures the voltage across the charge storage member and/or the
switch network. The process 600 begins when the controller 60
requests to initiate a charge operation at 602. At 604, the
controller 60 performs an initial voltage measurement at the
beginning of the charging operation. For example, the initial
voltage measurement may be across lines 55 and 57 (FIG. 2), or
nodes 302 and 304 (FIG. 3). Next at 606 the leakage detection
module 63 determines whether the initial voltage measurement
exceeds an initial charge threshold. In the example of FIG. 6, the
initial measurement represents a voltage and the initial charge
threshold a voltage, such as 12 volts. Optionally, the initial
measurement may represent a current measurement and the initial
charge threshold represents a current threshold. When the initial
charge measurement exceeds the initial charge threshold, this
indicates that the charge storage member is not loosing charge, and
instead already has an amount of stored energy that is indicative
of a non-leak condition. For example, it may have been determined
that when energy leaks from the charge storage member the level of
charge on the charge storage member does not rise to the initial
charge threshold (e.g. 12V). When the voltage measured on the
charge storage member exceeds 12 volts this is an indication that
the charge storage member is in an expected, fault-free condition.
Hence, no further leakage detection is warranted. Hence, when, at
the beginning of a charge request, the charge storage member
already has a voltage potential of over 12V, flow moves to 608
where charging is continued.
[0070] However, when the initial charge measurement is less than
the initial charge threshold (e.g. less than 12 volts), then this
is an indication that the charge storage member may potentially not
be operating in an expected, fault-free condition. Therefore,
further testing is warranted. Hence, flow moves along 610 to 614.
At 614, the controller 60 begins to attempt to charge the charge
storage member to a predetermined level, which may correspond to
the initial charge threshold (e.g. 12 volts). At 616, a delay is
introduced (e.g. 10 msec.) during which charge is applied to the
charge storage member. The delay may be for a predetermined or
programmable period of time. Alternatively, the delay may be for a
time determined by the controller 60 based on the condition of the
battery 110. After the delay at 616, the charging operation is
stopped at 618. At 620, another delay is introduced (e.g. 10 msec).
This second post-charge delay is set to afford the charge storage
member an opportunity to hold or loose its charge. When the IMD 10
is in a fault-free condition, the charge storage member will hold
the charge.
[0071] At 622, a leakage measurement (e.g. a voltage measurement)
is obtained by the leakage detection module 63. For example, the
leakage measurement may represent a voltage potential across the
lines 55 and 57, across the terminals of the charge storage member
and/or across the nodes 302 and 304 of the switch network 300. At
624, the leakage measurement is compared with a leakage threshold
(e.g. 8V). When the leakage measurement exceeds the leakage
threshold, it is determined that the charge storage member is
holding the charge in an expected fault-free manner. Thus, flow
moves to 626. However, when the leakage measurement does not exceed
the leakage threshold, it is determined that the charge storage
member has lost a portion of the prior charge applied thereto.
Hence, a current leakage condition exits and flow moves along 628
to 630 where a leakage identification error flag is set. Once the
flag is set at 630 various actions may be taken as described
throughout.
[0072] The process 600 may be repeated once every time the IMD 10
initiates a charging operation. Alternatively, the process 600 may
be repeated after a preset number of charging operations.
Alternatively, the process 600 may be repeated periodically (e.g.
one per day, once per week, etc.). The process 600 monitors the
rate of rise of the voltage on the charge storage member. The IMD
10 measures the voltage after a short initial-charge interval, such
as 20-50 msec., which occurs at 614 in FIG. 6. During the
initial-charge interval at 614 the rise in the voltage (charge
pattern) of a normally functioning IMD 10 should be approximately
20-50V. The rise in voltage of a normal charge pattern may be more
or less depending upon the capacitance of the charge storage
members and the charging current that the battery is able to
deliver. If the actual charge pattern has a voltage rise that is
slower than the expected charge pattern, then the IMD 10 may be
experiencing current leakage. The process 600 of FIG. 6 may be
implemented in the firmware of the IMD 10 without the addition of
new hardware components.
[0073] FIG. 4 illustrates a circuit diagram of an exemplary charge
storage member and leakage detection system 400 to detect current
leakage from the IMD 10 to surrounding excitable cardiac tissues by
sensing voltage in a charge storage member. The system 400
generally includes a charge (or energy) storage member 421, a
switch bank 426, a load 427 and a current sensing circuit 431, all
of which are configured to operationally connect to each other
under the general control of the controller 60 and under partial
control of the leakage detection module 63 of FIG. 2.
[0074] The charge storage member 421 includes multiple capacitors
402 and 404 that are chargeable through transformers 406 and 408 by
the battery 110. The switch bank 426 is generally configured to
form a H-shaped bridge or a H-bridge, in which four switches
452-455 are disposed along legs of the bridge (or switch bank) 426.
An external load 427 is illustrated. The external load 427
represents the tissue of interest (e.g., the heart or another
organ), proximate to which electrodes are positioned as illustrated
in FIGS. 1 and 3. The switch bank 426 connects and disconnects the
charge storage member 421 to connector terminals 423 and 425. The
connector terminals 423 and 425 are joined to one or more
electrodes. For example, the connector terminals 423 and 425 may
represent any of the terminals 40-58 in connector 43 of FIG. 2.
[0075] The current sensor circuit 431 is disposed along line 429
between the charge storage member 421 and the switch network 426.
The current sensor circuit 431 includes a resistive load 432
located along the line 429. The resistive load 432 is provided in
series with a positive or negative node of the switch network 426
which, when closed, becomes coupled to one of the connector
terminals 423, 425. The resistive load 432 forms a current sensing
resistor. A relatively low voltage potential is formed across the
resistive load 432 when leakage current flows in line 429.
[0076] A diode 433 is connected in parallel with the resistive load
432. When the diode 433 is forward biased, the diode 433 has a
maximum forward voltage drop of less than 2 Volts depending on the
amount of current flow. The forward biased diode then bypasses the
resistive load 432 when current flows in the direction of arrow A.
Thus, the diode 433 limits the amount of energy wasted by the
resistive load 432 to avoid any undue impact on the delivered
energy to the patient. A sensing circuitry 434 detects the voltages
at the input and output nodes of the resistive load 432. Current
flow along line 429 is unidirectional and thus only a single diode
433 is utilized. The diode 433 is rated to withstand the full
current capability of the IMD 10 such as a 50 Amp shock. The
resistive load 432 may be chosen to sense leakage current above a
value that will cause harm to the tissue of interest. For example,
a shocking lead may have an area of 4 cm.sup.2. When using the
criteria of 1.5 mA/cm.sup.2, then the resistive load 432 will be
chosen to detect a gross leakage of 6 milliAmps. For example, a
resistive load 432 of 100 ohms may be used. The current sensor
circuit 431 may be only turned on during a high voltage charging
operation.
[0077] In one embodiment, the sensing circuitry 434 may be a
comparator that produces a difference signal (denoted as signal
435). When the sensing circuitry 434 is a comparator, the signal
435 corresponds to the difference between the voltage across the
resistive load 432 and a preset voltage that corresponds to the
maximum allowable leakage current, both are inputs to the sensing
circuitry 434. When the sensing element 434 is a comparator, the
comparator is configured such that the signal 435 changes between a
logical high state and a logical low state. For example, the signal
435 may have the logical low state when no current flows in line
429. The signal 435 switches to the logical high state when current
begins to flow in line 429 in the direction of arrow A. The signal
435 is provided to the leakage detection module 63 (FIG. 3). The
leakage detection module 63 measures the signal 435 to determine
whether signal 435 is in a logical high state or in a logical low
state. For example, the leakage threshold may be satisfied by (e.g.
correspond to) one of the logical high and low states. The leakage
detection module 63 monitors the signal 435 to identify when
current is flowing. Current should be flowing during delivery of
therapy, but not between therapies. For example, when no therapy is
being delivered (therapy-free), the controller 60 instructs the
switch bank 426 to change to an open state and disconnect the
connector terminals 423, 425 from the charge storage member 421.
When in a therapy-free phase, no current should be flowing through
line 429. During the therapy-free phase, when the leakage detection
module 63 identifies current flow, this is an indication that
current leakage may be occurring.
[0078] In another embodiment, the sensing circuitry 434 may be an
analog to digital (A/D) converter that produces a digital data
value, as the signal 435 that corresponds to the voltage potential
across the resistive load 432. The digital data value, as signal
435, is supplied to the leakage detection module 63 in the
controller 60. The leakage detection module 63 analyzes the digital
data value to identify the level of leakage current.
[0079] FIG. 5 illustrates a circuit diagram of an exemplary leakage
detection system 500 formed in accordance with an alternative
embodiment. The leakage detection system 500 detects current
leakage from the IMD 10 to surrounding excitable cardiac tissues by
sensing current flow to the lead. The leakage detection system 500
generally includes a charge (or energy) storage member 521, a
switch bank 526, and a load 527, all of which are configured to
operationally connect to each other under the control of the
controller 60 and in part under the control of the leakage
detection module 63 of FIG. 2.
[0080] The external load 527 represents the tissue of interest
(e.g., the heart or another organ), proximate to which electrodes
are positioned as illustrated in FIGS. 1 and 3. The switch bank 526
connects and disconnects the charge storage member 521 to connector
terminals 523 and 525. The connector terminals 523 and 525 are
joined to one or more electrodes.
[0081] A current sensor circuit 531 is disposed along line 522
between the switch network 526 and the connector terminal 523.
Optionally, the current sensor circuit 531 could be disposed
between the switch network 526 and the connector terminal 525. The
current sensor circuit 531 includes a resistive load 532 located
along the line 522. The resistive load 532 is provided in series
with a positive or negative node of the switch network 526 which,
when closed, becomes coupled to the connector terminal 523. The
resistive load 532 forms a current sensing resistor. A relatively
low voltage potential is formed across the resistive load 532 when
current flows in line 522.
[0082] Diodes 529 and 533 are connected in parallel with the
resistive load 532. The diodes 529 and 533 are oriented in opposite
directions such that diode 529 is reverse bias (in an open circuit
state) when diode 533 is forward bias (in a closed circuit state).
In reverse, diode 529 is forward bias (in a closed circuit state)
when diode 533 is reverse bias (in an open circuit state). When
either of the diodes 529 or 533 is forward biased, the forward bias
diode 529 or 533 has a maximum forward voltage drop of less than 2
Volts depending on the amount of current flow. The forward biased
diode, either 529 or 533, then bypasses the resistive load 532 when
current flows in either direction through line 522. The diodes 529
and 433 limit the amount of energy wasted by the resistive load 532
to avoid any undue impact on the delivered energy to the
patient.
[0083] A sensing circuitry 534 detects the voltage across the
resistive load 532 and outputs a signal 535. The sensing circuitry
534 may represent a comparator or an analog to digital converter.
The signal 535 switches between logical high and low states when
the sensing element 534 is a comparator. The signal 535 represents
a digital data value of a measured voltage potential or current
flow when the sensing circuitry 534 is an A/D converter.
[0084] The leakage detection module 63 is also configured to
manipulate at least one of the switches 452-455 of the switch bank
426, thereby manipulating operational configurations of various
terminals such as terminals 40, 42-46, 48, 52, 54, 56, 58 of FIG.
2. Once the charge storage member 421 is charged for a preset
charging period, the leakage detection module 63 senses the voltage
of the charge storage member 421 and, when desirable, calculates
the rate of change of the voltage. The leakage detection module 63
compares the sensed voltage and the calculated rate of change in
the voltage to a preset threshold voltage and a preset threshold
rate of change (e.g., rise), respectively. When the sensed voltage
of the charge storage member 421 is found to be less than the
preset threshold voltage and/or when the calculated rate of change
in the voltage falls below the preset threshold rate, the leakage
detection module 63 identifies such behavior as a current
leakage.
[0085] Optionally, the leakage detection module 63 may suspend the
charger for a short interval (approximately <100 ms) and monitor
the voltage decay on the charge storage members 421. If the voltage
at the end of the short interval has decayed below a certain
threshold, then the behavior is identified to indicate current
leakage from the charge storage members 421.
[0086] In accordance with an embodiment, the controller 60 is
programmed to charge the charge storage member 421 for a preset
charging period, which is generally less than, about a hundred msec
and, more particularly, less than about 50 msec. However, the
charging period may vary according to various physiological or
pathological conditions of the patient's heart, and electrical
characteristics of the charge storage member 421 such as its
capacitance. At the end of the charging period, normally
functioning, non-leaking charge storage member 421 is charged
generally to a set range of, for example, 10 volts to 50 volts,
corresponding to a range of the rate of increase in voltage from
about 0-12 V in about 3 msec. Such a rate may generally be obtained
as the rate averaged over the beginning of the charging period. The
leakage detection module 63 then compares the sensed voltage to the
preset threshold voltage or compares the calculated rate of
increase or decrease in voltage with the preset threshold rate. The
leakage detection module 63 is generally configured to identify
current leakage upon detecting the sensed voltage that is less than
the preset voltage and/or upon detecting the calculated rate that
fails to reach the preset rate. It is understood that the preset
standard voltage of the charge storage member 421 as well as the
preset standard rate of increase in the voltage can vary with each
IMD. Advantageously, the foregoing exemplary leakage detection
system 400 can be algorithmically implemented in pre-existing IMDs
without adding new hardware. That is, the foregoing embodiment can
readily be practiced in pre-existing IMDs by reprogramming their
controllers to compare the sensed voltages to the preset standard
values and/or to compare the calculated rates of change in the
voltage of their pulse generators or other capacitors to the preset
standard rates.
[0087] When an IMD 10 is operating in a current leakage condition,
current flows from the IMD 10 to the tissue of interest whenever
the IMD 10 begins to charge the charge storage member. In certain
instances, the tissue of interest may be the heart and the lead may
be located in the right atrium or right ventricle. When left
unmanaged, the leakage current may be delivered to the atrium or
ventricle at a time in the cardiac cycle during which the atrium
and/ventricle are responsive to electrical stimulation. When
sufficient leakage current is delivered at a time when the atrium
and/ventricle are responsive to electrical stimulation, then the
leakage current may capture the myocardium similar to a pacing
pulse. It may be desirable to prevent the leakage current from
interfering with the normal sinus rhythm of the heart.
[0088] FIG. 7 illustrates an exemplary atrial cardiac event. FIG. 7
illustrates an atrial electrocardiogram (EGM) 710 aligned in time
with an atrium channel 714. When a P-wave 730 occurs, the atrium
enters an absolute atrium refractory period 722, followed by a
relative atrium refractory period 724. During the absolute
refractory period 722, the atrium is not sensitive to electrical
stimulation. The controller 60 times the start and end times for
the initial charge period to align with the refractory period 722.
The controller 60 defines an atrial charge window 736 that extends
from the P-wave for a period of time that is no longer than the
absolute refractory period 722. By aligning the atrial charge
window 736 with the absolute refractory period 722, the controller
60 avoids introducing charge into the charge storage member during
a time period when the atrium is sensitive to electrical
stimulation. If the atrium is refractory when the IMD 10 starts to
charge the charge storage member, then any leakage current that
might escape does not interfere with the atrium normal sinus
rhythm.
[0089] FIG. 8 illustrates an exemplary ventricular cardiac event.
FIG. 8 illustrates a ventricular electrocardiogram (EGM) 812
aligned in time with a ventricular channel 816. When an R-wave 834
occurs, the ventricle enters an absolute ventricular refractory
period 822, followed by a relative ventricular refractory period
824. During the absolute refractory period 822, the ventricle is
not sensitive to electrical stimulation. The controller 60 times
the start and end times for the initial charge period to align with
the refractory period 822. The controller 60 defines a ventricular
charge window 836 that extends from the R-wave for a period of time
that is no longer than the absolute refractory period 822. By
aligning the ventricular charge window 836 with the absolute
refractory period 822, the controller 60 avoids introducing charge
into the charge storage member during a time period when the
ventricle is sensitive to electrical stimulation. If the ventricle
is refractory when the IMD 10 starts to charge the charge storage
member, then any leakage current that might escape does not
interfere with the ventricles normal sinus rhythm.
[0090] FIG. 9 illustrates a charge timing process 900 implemented
in accordance with an embodiment to time the initial charge period
to overlap the atrial or ventricular refractory period 722 or 822.
At 902, the IMD 10 detects and analyzes cardiac signals. Among
other things, the IMD 10 detects the R-R interval and analyzes the
cardiac signals to determine if an arrhythmia is present. If
certain arrhythmias are present, then the IMD 10 will determine
that a high voltage shock should be delivered. At 904, the IMD 10
determines whether the cardiac signal indicates that a HV shock is
needed. If no shock is needed, flow returns along path 906. When an
HV shock is needed, flow moves along 908. At 910, the controller 60
identifies the start time of the refractory period of interest. For
example, the controller 60 may identify the start of the atrial
refractory period 722 when it is desirable to charge the charge
storage member during atrial activity. Optionally, the controller
60 may identify the start of the ventricular refractory period 822
when it is desirable to charge the charge storage member during
ventricular activity. The start times for the atrial and
ventricular refractory periods 722 and 822 may be calculated from
the P-wave and the R-wave, respectively.
[0091] Next, at 912, the controller 60 sets the start and end times
for the initial charge period to align with the corresponding
refractory period (722 or 822). At 914, the controller 60 initiates
a charging operation. By timing the charging period with the
refractory period, the IMD 10 limits any adverse effects that may
result when current leaks from an IMD. The charge storage member
421 is charged during a cardiac refractory period in which at least
the majority of the excitable cardiac tissues become immune to
stimuli. Therefore, the controller 60 is preferably configured to
synchronize the charging period of the charge storage member 421
with the atrial and/or ventricular events.
[0092] The controller 60 is configured to synchronize the charging
periods of the charge storage member 421 within a ventricular
charge window 836. The ventricular charge window 836 extends for a
predetermined or programmable period, i.e., approximately 0 msec to
40 msec following the detection of the R-wave, but still within the
absolute ventricular refractory period 822. Advantageously, the
excitable cardiac tissues do not respond to the stimulus during the
ventricular charge window 836 because the ventricles are refractory
and thus are not responsive to stimuli such as a stimulus resulting
from a current leakage.
[0093] The same applies to the atrial however because the
ventricular chamber takes priority in synchronization it may be
preferred to first synchronize to an R-wave and then make charge
out of the atrial vulnerable period.
[0094] Numerous variations of the present systems and methods exist
without deviating from the scope of the present invention. For
example, multiple leakage detection systems may be provided at
different locations of the IMD 10 to detect leakage currents at
these locations. For example, multiple switch banks may be
implemented in parallel such that each switch bank may be disposed
between the charge storage member and each implanted electrode. The
controller may measure the voltage of the charge storage member,
the rate of increase in the voltage, and/or the current flowing
through respective current sensing members. When a leakage current
exceeding the preset limit is detected, the controller terminates
the charge supply to the leaking electrodes, while maintaining
normal operation of the non-leaking electrodes.
[0095] FIG. 10 illustrates a post-leak assessment process 1000
performed by the controller 60 after leakage has been confirmed in
accordance with an embodiment. The process 1000 begins when a leak
is detected at 1002. Once leakage is detected, the leakage
detection module 63 issues a visual and/or an audible warning
signal at 1004 to the patient. The warning may also be physical
such as a periodic vibration. At 1006, the leakage detection module
63 may determine whether the switch network 426 can be manipulated
to minimize the impact of leakage. For example, the switch network
426 may be adjusted by opening or closing all, or one or more of
the switches 452-455. Alternatively, if it is determined that only
one switch is in-operative, the switch network 426 may no longer be
able to deliver biphasic shocks. However, the switch network 426
may be able to deliver mono-phase shocks. Thus, the switch network
426 may be adjusted to a state where only mono-phase shocks can be
delivered thereafter.
[0096] At 1008, the IMD 10 logs into memory any error information,
the leakage measurements, IMD 10 status parameters, the state of
various components within the IMD 10 at the time of leakage
detection, recent cardiac activity, and the time at which leakage
was detected. At 1010, the IMD 10 determines whether the state of
the switch network 426 warrants one or more remedial actions. If
the state of the switch network 426 does not warrant any remedial
action, flow moves along 1012 and is done. However, if a remedial
action is warranted, flow moves along 1014 and the IMD 10 disables
the charging capability at 1016. At 1016 the charging and/or
discharging process is terminated for the charge storage member
421, in order to eliminate the detected current leakage. When
desirable, the leakage detection module 63 may also be configured
to terminate a specific operation or the entire operation of the
IMD 10 until a patient or an operator takes proper corrective
actions. Accordingly, the leakage detection system 400 can limit
the leakage of current to the patient even after the destruction of
various electrodes 22, 23, 27, 28, 32, 34, 36, 38.
[0097] The leakage detection systems described herein generally
seek to protect the patients from hazardous "strong" current
spuriously leaking from the implanted electrodes. Other capacitors
of the IMD, however, may also be utilized in detecting leakage of
weak current which may gradually deplete the power cells of the
IMD. Such capacitors may be selected to have high or low
capacitance. Advantageously, upon selecting a desirable capacitor
of the IMD, the foregoing leakage detection system and methods
therefore can readily be implemented to the IMD 10 by reprogramming
its controller to sense the voltage of the capacitor or to
calculate the rate of change of its voltage. When desirable, an
additional voltage sensor may be provided to sense the voltage of
the capacitor. In an alternative embodiment, one or more capacitors
may be added at desirable locations of the IMD, with the controller
60 sensing the voltage and/or rate of change of the added
capacitors.
[0098] In yet another embodiment, the IMD 10 may be configured to
minimize further leakage by suspending charging of the IMD,
terminating the charging process of its charge storage member, and
terminating operation of the IMD 10 until proper remedial action is
taken. In particular, the controller may be configured to raise a
warning flag, when the sensed voltage of the charge storage member
after the pre-selected charging period does not reach the preset
voltage or drops to a certain threshold after a wait period, when
the sensed rate of increase in the voltage of the charge storage
member is less than the preset rate, or when the leakage current
sensed through the current sensing member exceeds the preset value.
Upon detecting the warning flag, the IMD 10 which is provided with
various warning systems configured to issue audio or vibratory
warning signals to warn the patient or operator of current leakage,
may issue various visual and/or audible warning signals to the
patient and/or operator and/or log the error in its memory for
future presentation to the following physician.
[0099] Optionally, the current sensor circuit may be at other
locations within the circuitry of the IMD 10 as long as such the
current sensor circuit senses the current flow between the charge
storage member and the electrodes 28, 36, 38. In addition, because
most current leakages tend to occur at, or near such electrodes,
the current sensor circuit is generally placed in series with the
leads 20, 24, 30.
[0100] In certain instances, the current leaking from a HV
electrode may be less than 1 uA while the current leaking from a
non-HV electrode being less than 0.1 uA. Accordingly, the
controller 60 may be arranged to detect and to limit the current
leakage that exceeds a predetermined limit. When current leakage is
detected at such a low level as to pose no concern to the safety of
the patient, in such instances the IMD 10 may not take an immediate
corrective action. Instead, the low level current leakage may be
permitted to continue without disabling charge. Even during low
level current leakage, it may be desirable to warn the patient or
notify a physician and to log certain information regarding the
condition of the IMD.
[0101] Thus, various systems and methods therefore using leakage
detecting and warning systems in implantable medical devices have
been described in which currents and/or voltages are measured at
various locations of such devices in order to detect leakage of
current and patient warning signals are issued. While detailed
descriptions of the specific embodiments of the present invention
have been provided, it would be apparent to one of ordinary skill
in the relevant art that numerous variations of the systems and
methods described herein may be possible in which the concepts of
the present invention may readily be applied and are not intended
to be exclusive.
[0102] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. In addition, many modifications may be
made to adapt a particular situation or material to the teachings
of the invention without departing from its scope. While the
dimensions, types of materials and coatings described herein are
intended to define the parameters of the invention, they are by no
means limiting and are exemplary embodiments. Many other
embodiments will be apparent to those of skill in the art upon
reviewing the above description. The scope of the invention should,
therefore, be determined with reference to the appended claims,
along with the full scope of equivalents to which such claims are
entitled. In the appended claims, the terms "including" and "in
which" are used as the plain-English equivalents of the respective
terms "comprising" and "wherein." Moreover, in the following
claims, the terms "first," "second," and "third," etc. are used
merely as labels, and are not intended to impose numerical
requirements on their objects. Further, the limitations of the
following claims are not written in means--plus-function format and
are not intended to be interpreted based on 35 U.S.C. .sctn.112,
sixth paragraph, unless and until such claim limitations expressly
use the phrase "means for" followed by a statement of function void
of further structure.
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